Chapter II: Assembler
Chapter goal:
Overview:
 Introduce the fundamental  Basic Assembler Functions
functions that any
assembler must perform.  Machine-Dependent
Assembler Features
Assign machine address
 Translate mnemonic
operation codes to machine  Machine-Independent
Assembler Features
language equivalents.

 Assembler Design Options
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Basic Assembler Functions
(Using SIC as an Example)
 Assembler directives:
 START :
Specify name and starting address for the program
 END : …
 BYTE :
Generate character or hexdecimal constant
 WORD:
Generate one-word integer constant
 RESB :
Reserve the indicated number of bytes for a data area
 RESW : …
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Example of a SIC Assembler Language Program
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Example of a SIC Assembler Language Program (cont.)
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Example of a SIC Assembler Language Program (cont.)
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A Simple SIC Assembler
 The translation steps
 Convert mnemonic operation codes to their machine language
equivalent.

Convert symbolic operands to their equivalent machine
addresses.

Build the machine instructions in the proper format.

Convert the data constants specified in the source program into
their internal machine representations.

Write the object program and the assembly listing.
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Output: the object program
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The Object code for the above program
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The Object code for the above program (cont.)
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The Object code for the above program (cont.)
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The Format for Object Program
 The object program will later be loaded into
memory for execution.
 Three types of records for object program format
Header: contains the program name, starting address,
and length.
 Text: contains the translated instructions and data of
the program
 End: marks the end of the object program and
specifies the address in the program where
execution is to begin.

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The Format for Object Program (cont.)
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The object program
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Two Passes of our Simple Assembler
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The Data Structures
 Two major data structures:
 Operation code table (OPTAB)
 Symbol table (SYMTAB)
 Note: SYMTAB is usually organized as a hash table
for efficiently of insertion and retrieval.
 Location counter (LOCCTR)
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The Algorithm (Pass 1)
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The Algorithm (Pass 2)
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Machine-Dependent Assembler Features
(using SIC/XE as an example)
 Addressing modes
 Immediate addressing modes:
COMP #0
 Indirect addressing:
J @RETADR
 The extended instruction format
+LDT #4096

Most of the register-to-memory instructions are assembled
using either program-counter relative or base relative
addressing. If either program-counter relative nor base relative
addressing can be used, then the 4-byte (Format 4) must be
used..
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Example of a SIC/XE Assembler Language Program
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Example of a SIC/XE Assembler Language Program
(cont.)
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Example of a SIC/XE Assembler Language Program
(cont.)
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Output: the object program
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The Object code for the above program
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The Object code for the above program (cont.)
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The Object code for the above program (cont.)
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Program Relocation
 An object program that contains the information necessary to
perform this kind of modification is called a relocatable program.
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Program Relocation (cont.)
 We can solve the relocation problem in the following
way:
 1. When the assembler generates the object code for the JSUB
instruction we are considering, it will insert the address of
RDREC relative to the start of the program. (This is the reason we
initialized the location counter to 0 for the assembly)
 2. The assembler will also produce a command for the loader,
instructing it to add the beginning address of the program to the
address field in the JSUB instruction at load time.
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Program Relocation (cont.)
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Program Relocation (cont.)
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Machine-Independent Assembler Features
 Literals
 Symbol-Defining Statements
 Expressions
 Program Blocks
 Control Sections and Program Linking
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Literal
 It is often convenient for the programmer to be able to write the
values of a constant operand as a part of the instruction that uses
it. Such an operands is called a literal.
 E.g., (In Fig 2.9)
 45
 215
001A ENDFIL
1062 WLOOP
LDA =C’EOF’
TD
=X’05’
032010
E32011
 The difference between a literal and an immediate operand. With
immediate addressing, the operand value is assembled as part of
the machine instruction. With a literal, the assembler generate the
specified value as a constant at some other memory location.
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Literal (cont.)
 Literal pools: Normally literals are placed into a pool at the
end of the program. The assembly listing of a program containing
literals usually includes a listing of this literal pool, which shows
the assigned addresses and the generated data values.
 The assembler directive LTORG is used for creating the literal
pool.
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Program demonstrating additional assembler features
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Program demonstrating additional assembler features (cont.)
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Program demonstrating additional assembler features (cont.)
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The above program with object code
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The above program with object code (cont.)
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The above program with object code (cont.)
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Symbol-Defining Statements
 Most assembler provides an assembler directive that allows the
programmer to define symbols and specify their values.
 The assembler directive : EQU
 E.g.,
symbol
EQU
value
 Usage sample:
+LDT
#4096
+LDT
MAXLEN
#MAXLEN
EQU 4096
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Symbol-Defining Statements (An example…)
 STAB
 FLAGS
RESB 1100
EQU
EQU
EQU
 LDA
VALUE,X
 SYMBOL
 VALUE
STAB
STAB+6
STAB+9
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Expressions
 Assembler generally allow arithmetic expressions formed
according to the normal rules using the operators +, -, * , and /
 E.g.,
MAXLEN
EQU
BUFEND-BUFFER
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Program Blocks
 The source program logically contained subroutines, data areas,
etc. However they were handled by the assembler as one entity,
resulting in a single block of object code.
 Note:
The term program blocks refer to segments of code that are
rearranged within a single object program unit, and control
section to refer to segments that are translated into independent
object program units.
 The assembler directive USE indicates which portions of the
source program belong to the various blocks.
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Example of a program with multiple program blocks
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Example of a program with multiple program blocks
(cont.)
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Example of a program with multiple program blocks
(cont.)
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The above program with object code
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The above program with object code (cont.)
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The above program with object code (cont.)
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Program Blocks
 Pass 1
Use separate location counter for each program block.
 Pass 2
The assembler needs the address for each symbol relative to
the start of the object program.
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The object program
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The loading processes
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Control sections and program linking
 A control section is a part of the program that maintain its identity
after assembly; each such control section can be loaded and
relocated independently of the others.
 Note:
1. The assembler has no idea where any other control section will
be loaded at execution time.
2. The reference between control sections are called external
reference .
 Two assembler directive:
1. EXTDEF
2. EXTREF
: defined the external symbol that may be used
by other sections.
: named the symbols that are used in this
control section and defined elsewhere.
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Illustration of control sections and program linking
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Illustration of control sections and program linking
(cont.)
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Illustration of control sections and program linking
(cont.)
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The above program with object code
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The above program with object code (cont.)
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The above program with object code (cont.)
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Control sections and program linking (cont.)
 The two new record types are Define and Refer.
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The object program
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Assembler Design Options
– One-pass Assembler
 Main problem:
 One need to solve the forward reference problem.
 Solution:
 Require all such areas be defined in the source program before
they are referenced.
 In order to reduce the size of the problem, many one-pass
assemblers prohibit forward reference to data items.
 Usually one-pass assembler generate object code in
memory for immediate execution. No object program is
written out, and no loader is needed.
--------- load-and-go assembler.
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Assembler Design Options
– One-pass Assembler (cont.)
 If an instruction operand is a symbol that has not
yet been defined, the operand address is omitted
when the instruction is assembled.
 The address of the operand field of the instruction that
refers to the undefined symbol is added to a list of forward
references associated with the symbol table entry.
 When the definition for a symbol is encountered, the
forward reference list for that symbol is scanned, and the
proper address is inserted into any instructions previously
generated.
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Sample program for a one-pass assembler
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Sample program for a one-pass assembler (cont.)
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Sample program for a one-pass assembler (cont.)
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Object code in memory and symbol table entries
for above program (after scanning line 40)
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Object code in memory and symbol table entries
for above program (after scanning line 160)
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Object program from one-pass assembler for
above program
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Assembler Design Options
– Multi-pass Assembler
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Example of multi-pass assembler operation
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Example of multi-pass assembler operation (cont.)
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Example of multi-pass assembler operation (cont.)
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Example of multi-pass assembler operation (cont.)
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Example of multi-pass assembler operation (cont.)
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